Observable features of GW170817 kilonova afterglow

The neutron star merger, GW170817, was followed by an optical-infrared transient (a kilonova) which indicated that a substantial ejection of mass at trans-relativistic velocities occurred during the merger. Modelling of the kilonova is able to constrain the kinetic energy of the ejecta and its characteristic velocity but, not the high-velocity distribution of the ejecta. Yet, this distribution contains crucial information on the merger dynamics. In this work, we assume a power-law distribution of the form E(> βΓ) ∝ (βΓ)^-α for the energy of the kilonova ejecta and calculate the non-thermal signatures produced by the interaction of the ejecta with the ambient gas. We find that ejecta with minimum velocity β₀ ≃ 0.3 and energy E ∼ 10⁵¹ erg, as inferred from kilonova modelling, has a detectable radio, and possibly X-ray, afterglow for a broad range of parameter space. This afterglow component is expected to dominate the observed emission on a time-scale of a few years post-merger and peak around a decade later. Its light curve can be used to determine properties of the kilonova ejecta and, in particular, the ejecta velocity distribution α, the minimum velocity β₀, and its total kinetic energy E. We also predict that an afterglow rebrightening, that is associated with the kilonova component, will be accompanied by a shift of the centroid of the radio source towards the initial position of the explosion.